1. Field of the Invention
The present invention relates to thermal printers, and more particularly a thermal printer for carrying out a printing operation with a thermal head pressed upon a platen roller.
2. Description of the Prior Art
In this type of thermal printer, the thermal head is placed in pressure contact with recording paper on the platen roller with an ink ribbon disposed therebetween for carrying out a printing operation. The thermal head is moved away from the platen roller when not printing to allow the ink ribbon to be fed and the recording paper to be returned.
A typical example of the mechanism for moving the thermal head into and out of pressure contact is disclosed in Japanese Patent Publication Kokai No. 60-73878. This prior art mechanism includes a spring for urging the thermal head away from the platen roller, and a solenoid for driving the thermal head into pressure contact with the platen roller against the force of the spring. During printing the solenoid is activated to press the thermal head upon the platen roller against the force of the spring. When not printing, the solenoid is de-activated to allow the thermal head to be separated from the platen roller under the force of the spring. According to this prior art, the solenoid must be activated throughout a printing operation, which involves a wasteful consumption of power. Furthermore, since the solenoid is used, impulsive sounds are produced when the solenoid turns on and off. This is a serious drawback because low noise is a desired feature of thermal printers.
A construction intended for solving this problem has been disclosed in Japanese Patent Publication Kokai No. 61-130076. This known construction comprises a stepper motor and an asymmetrical cam driven by the motor. The cam is stopped at selected positions under control by the stepper motor to move a thermal head into and out of pressure contact with a platen roller.
Although this construction does not produce impulsive sounds since a solenoid is not employed, the use of the stepper motor results in a high manufacturing cost.